Typically, drilling rigs at the earth surface are used to drill lengthy boreholes into the earth to reach the location of subsurface oil or gas deposits and establish fluid communication between the deposits and the surface via the borehole. Downhole drilling equipment may be directed or steered to the oil or gas deposits using well-known directional drilling techniques, which may rely on the direction and orientation of downhole survey instruments that can be monitored at survey locations along the borehole.
Surveying of boreholes is typically performed by utilizing downhole survey instruments such as, for example, accelerometers and magnetometers coupled within a bottom hole assembly (BHA). The BHA is typically coupled in the drill string (e.g., the drill pipe or the drill collars) above the drill bit. The survey instruments may be used to measure the direction and magnitude of the local gravitational and magnetic field vectors to determine the azimuth and the inclination of the borehole at each survey location within the borehole. The survey measurements may be performed during drilling using a process commonly referred to as measurement while drilling (MWD). Generally, separate borehole surveys are conducted at the survey locations along the borehole when drilling is stopped or interrupted to couple additional stands of drill pipe to the drill string at the surface.
The direction of a drilled borehole within any segment of the borehole is usually determined by the method of drilling and the arrangement of the drilling equipment used to drill the segment of the borehole. For directional drilling using a bent stub and a mud motor, two known methods of drilling produce distinctive borehole trajectories. One known method referred to as rotating involves the rotation of the entire drill string, including the BHA. In this method, the bent stub is in straight line borehole trajectory. Although deviations from a true linear trajectory typically exist due to gravity, misalignment of equipment, etc.
A second known method of drilling referred to as sliding has the bent sub in a deployed or angular position to selectively adjust the angular position of the bit shaft relative to the drill collar. Using the sliding method, the drill bit is rotated by the mud motor instead of by the rotation of the drill string. Sliding produced a drilled borehole having a curved or generally arc-shaped trajectory. In practice, sliding produces boreholes that deviate from a true arc-shaped trajectory for the same reasons that rotating drilling processes produces boreholes that deviate from a true linear trajectory.
the rotation applied to a drill bit and the resulting torque (torque-on-bit or TOB) are important data that can be used to determine drill bit wear and drilling direction. However, during either rotating or sliding drilling there is usually some inefficiency associated with transmitting rotational torque to the drill bit. This inefficiency is commonly called drag, which may be defined as a retarding force exerted on a moving body by a medium. Surface measurements of TOB may not be accurate because factors such as, for example, borehole curvature, hole deformation and packing of stabilizers all contribute to drag that cannot be readily determined at the surface.
Various systems have been devised for conducting downhole measurements and transmitting these measurements uphole to the surface during drilling. One known system measures torque using string gages attached to a drill collar. However, signals produced by the bending of the collar may be larger that the torque signal and induce drift in the strain gages. Additionally, the relaxation of stresses in the drill collar can produce signals as large as the torque to be sensed by the strain gages.
Another known system is a wireline tool that includes one or more survey probes suspended by a cable and raised and lowered into and out of the borehole. A free part indicator tool probe can measure the angular and axial displacement between two anchored sections of the boreline tool, but such a probe cannot be utilized during drilling to make reliable measurements.
Piezo-magnetic sensors have also been proposed for making downhole MWD, but such sensors have limitations similar to those of strain gages. Additionally, the crushing and grinding of the drill bit against rock at the bottom of the borehole, the engagement of the drill string with the surfaces of the borehole, and the stresses experienced by the joints of the drill pipe and the drill collars, all combine to produce noise, shock and vibrations that corrupt measurements of the earth's magnetic and gravitational fields, thereby rendering such downhole measurements or data unusable for determining accurately the characteristics of the borehole.